As is known, polyisocyanates having biuret structure are obtained by reacting three moles of an organic di-isocyanate with one mole of an adduct-producing agent.
The term "organic diisocyanate", as used in the present specification and claims, is intended to mean the compounds having general formula: EQU OCN--R.sub.1 --NCO (II)
wherein R.sub.1 is aliphatic, cycloaliphatic or aliphatic-cycloaliphatic radical which can or cannot be substituted with halogen, such as chlorine, NO.sub.2, and alkyl radical, an alkoxy radical, non-reactive oxydryl groups, etc.
Examples of organic diisocyanates are: ethylidendiisocyanate, butylene diisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate, cyclopentylene-1,3-diisocyanate, cyclohexylene-1,4-diisocyanate, cyclohexylene-1,2-diisocyanate, hexahydroxy xylylene-diisocyanate, dichloro-hexamethylenediisocyanate, dicyclo-hexyl-4,4'-diisocyanate, 1,2-di(isocyanatomethyl)cyclobutane, 1-methyl-2,4-diisocyanate-cyclohexane, 1-methyl-2,6-diisocyanate cyclohexane, etc.; aliphatic diisocyanates containing ethereal groups such as 1,3-bis-(.gamma.-isocyanatopropoxy)-2,2-dimethyl-propane, etc.
Suitable adduct-producing agents are: water, water contained in compounds in the form of crystallization water or at the nascent state, formic acid, chlorohydrates, hydrate alcohols, monovalent tertiary alcohols, dicarboxylic acids which can be changed into their anhydrides such as oxalic acid, maleic acid, salicylic acid, etc., sulphide acid, primary and secondary amines, diamines, polyamines, etc.
Processes for the production of polyisocyanates having biuret structure using the above-mentioned adduct-producing compounds are described in U.S. Pat. Nos. 3,124,605, 3,350,438, 3,358,010, 3,392,183, 3,862,973, 3,896,154, 4,051,165, 4,147,714, 4,176,132, 4,218,390, 4,320,068, etc.
As is known, the reaction of an organic diisocyanate having formula (II) with an adduct-producing agent leads to the formation of an ureic bond (R.sub.1 --NH--CO--NH--R.sub.1) which further reacts with isocyanic groups to form a tri-or poly-functional polyisocyanate.
The reaction can be schematized as follows when water is used as adduct-producing agent: ##STR2##
The thus formed polyisocyanate having biuret structure is soluble in common organic solvents such as toluene, xylene, acetic acid esters, etc.
The hereinabove reported reactions 1-4 are consecutive, but bifunctional urea or oligomers containing ureic bonds which form during the reactions can be solid or slightly soluble in the reaction medium and/or with the thus-formed polyisocyanate product. This makes the final product muddy and involves problems of deposit and clogging in the production plant.
However, while, on one hand, it is desirable that the ureic groups (NH--CO--) react with the isocyanic groups (OCN--), on the other hand, a polyisocyanate devoid of free ureic groups has high viscosity specially when its molecular weight is high.
A very viscous polymer is scarcely miscible in other resins or in solvents and, therefore, it is not suitable for being used, for example, in paint without negatively affecting the filmogenic and physical properties of the paint itself.
It has been proposed to use an excess of the organic diisocyanate in respect to the adduct-producing agent to produce polyisocyanates having biuret structure, devoid of free ureic groups and with low molecular weight.
Thus, U.S. Pat. No. 3,903,127 describes a process for the preparation of polyisocyanates with biuret structure wherein the molar ratio diisocyanate/adduct-producing agent is at least 11:1. This solution of the problem is not free of inconveniences due both to the stripping of the free diisocyanate and to the eventual thermal polymerization of the excess diisocyanate with formation of colored products such as uretidions, isocyanates, carbodiamidic bonds, etc.
Another inconvenience which arises during the preparation of polyisocyanates having biuret structure is that the adduct-producing agents are generally slightly soluble or completely insoluble in diisocyanate. Thus, for example, when water is used as adduct-producing agent, emulsions are obtained due to the low miscibility of water in organic diisocyanate. The reaction between water and the diisocyanate monomer dissolved in the aqueous phase leads to the formation of polyureas which precipitate as by-product. The precipitation of polyureas is not only economically disadvantageous, but it reduces the commerical value of the final polyisocyanate.
The amount of polyureas as by-product varies according to temperature, stirring speed, diisocyanate/water ratio, etc., but generally it always exceeds 0.5% by weight with respect to the final polyisocyanate. This amount of polyurea can be minimized by adding 40 moles or more of diisocyanate monomer per mole of water. A high excess of diisocyanate, however, does not completely eliminate the formation of polyureas and, in addition, it involves the problem of eliminating the large excess of non-reacted monomer.
In order to prevent the formation of polyureas as byproduct, it is proposed, in U.S. Pat. 4,028,392 to prepare a polyisocyonic prepolymer by reacting a diisocyanate with water in an organic hydrophilic solvent capable of dissolving both the water and the organic diiscyanate to form a homogeneous phase. The hydrophilic solvents proposed in said patent must not be reactive with the isocyanate group and are selected from the esters of carboxylic acids, esters-amides of phosphoric acid, ketones, nitrilies, ethers, etc., such as methyl-cellosolve acetate, cellosolve acetate, methyl-butyl-ketone, trimethylphosphate, di-methyl-formamide propionitrile, adiponitrile, etc.
However, this technique too is not free of drawbacks due to the removal of said solvents at the end of the reaction. In fact, incomplete removal of the solvent affects the physical properties of products such as varnishes or paints, obtained from polyisocyanates.